There has hitherto been known a sensor in which, as shown in FIG. 9, a positive electrode 31 and a negative electrode 32 are provided on the surface of a substrate 30 formed of a solid electrolyte, such as zirconia, and a gas inlet from outside to the negative electrode 32 is formed as a small-sized hole 33 for limiting the gas diffusion towards the negative electrode 32, the gas thus limited being ionized on the surface of the negative electrode 32 so as to be migrated through the inside of the solid electrolyte substrate 30 to produce a limiting current based on which to detect the gas concentration. This sensor is herein termed as a limiting current type gas sensor. There has also been known a sensor of the type in which the negative electrode 32 itself is of a porous structure so that the pores of the negative electrode are used simultaneously as the small-sized hole for limiting the gas diffusion. This structure is superior in that the sensor is of a simple planar structure.
If the apparatus for detecting oxygen or water vapor employing a limiting current type sensor is used for a prolonged period of time, the limiting current characteristics of the sensor versus applied voltage are deteriorated depending on the measurement atmosphere, thus increasing measurement errors. With this in mind, there has been proposed a method in which two sensor elements having different values of the applied voltages at which the limiting current appears are incorporated in one sensor and the current ratio between the two is measured for giving judgment on the degree of deterioration based on the extent of deviation of the measured ratio from the initial value, as disclosed in JP Patent kokai JP-A-4-264250. There has also been proposed a method in which the humidity is found from a ratio of the first limiting current value limited depending on the concentration of oxygen in the measurement atmosphere to the second limiting current value limited depending on the moisture concentration for reducing the error despite sensor deterioration, as disclosed in JP-Patent Kokai JP-A-4-50763 (U.S. Pat. No. 5,281,314).